The NSS Bulletin
- ISSN 1090-6924Volume 37 Number 3: 43 - July 1975A publication of the National Speleological Society

Firn Caves in the Valcanic
Craters of Mount Rainier, WashingtonEugene P. Kiver and William K. Steele

Abstract

Sub-ice fumaroles and warm
air currents form and maintain over 2 km of cave pasage beneath ice filling
the summit craters of Mount Rainier. Passage size increases from 1970 to 1973
indicate recent, minor, heat flow increases. Large heat flow decreases would
allow plastic flowage to close passages and large increases would produce enlargement,
collapse, and large crater lakes. Complete melting of summit ice would produce
about 1.1 billion liters of water in the west crater and 7.4 billion liters
in the east crater, creating a serious potential geological hazard.

The caves are called firn
caves because ice density ranges from 0.55 to 0.81 gm/cm3. Ice tempteratures
as low as -10°C partly account for low densities, high viscosity ice, and
open passages as deep as 100 m below the east crater snow surface.

Subsising ice replaces walls
and ceilings that are melted back 2.0 to 3.5 m/year. Subsidence, melting, and
snowfall are in dynamic equilibrium. Thus, cave dimensions and the snow surface
within the crater remain relatively constant. The discovery of a climber's glove
and the debris from a 1959 expedition shows that subsiding ice reaches deeper
cave areas in a few decades.

Entrance passages lead down
the crater slope and perimeter passages parallel elevations contours. An east
crater perimeter passage 50 m below the surface in its central part is 915 m
long and winds three-fourths of the way around the crater. Narrow passages lead
downward from the main perimeter passage in each crater to large grottos. A
40 m long lake in the west crater grotto is the highest crater lake in North
America.